Membranes play a crucial role in almost all cellular phenomena, yet we know little about the molecular organization of these structures or the functions which they carry out. The lipid component of many natural membranes has been well characterized, but we know little about the protein component mainly because of the insoluble nature of these proteins which makes them difficult to purify and study. However, it is only the protein component of membranes which can confer the high degree of specificity necessary to carry out the complex functions associated with biological membranes. In addition, proteins must play an important role in membrane structure. Methods of affinity chromatography using CDP-diglyceride Sepharose have been developed in this laboratory to purify membrane bound enzymes of phospholipid metabolism. By using affinity chromatography the phosphatidylserine synthetase has been purified to homogeneity and the phosphatidylglycerophosphate synthetase to near homogeneity. The physical, chemical and enzymological properties of these purified enzymes will be characterized in order to better understand membrane function and membrane biogenesis as well as the control of cell growth.